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    <script>
        var preorderTraversal = function(root) {
            const res = [];
            var func = function(root) {
                if (root) {
                    res.push(root.val);
                    if (root.left) {
                        func(root.left);
                    }
                    if (root.right) {
                        func(root.right);
                    }
                }
            }
            func(root)
            return res
        };
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